1. Field of the Invention
The present invention relates to a wire shaped electron source for radiating electrons and, more particularly, to a wire shaped electron source which functions as a filament cathode for use in a flat panel display device.
2. Description of the Prior Art
In general, wire shaped electron sources are commonly used as filament cathodes in fluorescent display devices, thin tube CRT display devices, and similar products. As shown in FIG. 4, a wire shaped thermionic cathode 42 comprising an electron emission oxide material 41 applied around a tungsten core 40 with a diameter of approximately 20 .mu.m is widely used in the above display devices. The wire shaped thermionic cathode 42 is riding on supports 43 in a tensioned state. The tungsten core 40 is used to heat the wire-shaped thermionic cathode 42 to emit electrons from the electron emission material 41.
In a display device having a large display area, it is possible to use a long wire-shaped thermionic cathode 42 in a tensioned state, but the brightness of the emitted light from the phosphors varies when the wire-shaped thermionic cathode 42 vibrates, as in the manner described in the Japanese Laid-open Patent Publication No. 5424570, published Feb. 13, 1979.
To prevent such vibration, a center support Sc is provided at the center portion of the long wire shaped thermionic cathode 42. However, the center support Sc contacts the wire shaped thermionic cathode 42 and scrapes off the electron emission material 41 laminated on the cathode 42. The scraped off particles of the electron emission oxide material become dust, which adheres to other components, such as an electron beam extraction electrode 44, degrading the ability of the emission of electrons from the cathode 42.
As shown in FIG. 5, to prevent the vibration of the cathode and the scraping off of the electron emission material, a spiral wire shaped thermionic cathode is proposed, such as described in the Japanese Laid-open Patent Publication No. 61243633, published Oct. 29, 1986. A spiral wire shaped thermionic cathode 53 is arranged between a back electrode means 51 and an electron beam extraction electrode 52. The wire shaped thermionic cathode 53 is formed by the application of an electron emission oxide material 56 over a heater wire comprising a tungsten core 54 and a tungsten coil core 55. The wire shaped thermionic cathode 53 is processed through a die to remove the electron emission material 56 from the outer surface of the coil core 55. The resulting cathodes 53 are then tensely stretched with slight contact with spacers 57 formed on the electron beam extraction electrode 52.
However, in the prior art wire shaped thermionic cathode 53 as thus described, because the spiral core 55 is made of the same tungsten metal wire as the straight core wire 54, the electron emission material 56 adheres not only to the straight core wire 54 but also to the outer surface of the spiral core 55 when it is applied with electrodeposition or a spray method used in heater wire production. As a result, to remove the electron emission material 56 adhering to the outer surface of the spiral core wire 55, the wire shaped thermionic cathode 53 is passed through a die.
It is, however, difficult to sufficiently remove the electron emission material 56 from the outer surface of the spiral core wire 55, resulting in various problems, such as insufficient removal of the electron emission material 56 from the spiral core wire 55 or too much removal of the electron emission material 56 from the straight core wire 54 to cause uneven emission of electrons from the wire-shaped thermionic cathodes 53.
The object of the present invention is therefore to provide a wire-shaped electron source which will not make any vibrations and which can maintain the electron emission oxide material on the electron source to produce no dust or the like.
In Japanese Laid-open Patent Publication No. 2270252, published Nov. 5, 1990, an electron source as shown in FIG. 6 is disclosed, which has a metallic or insulated filament 2 wound in a spiral pattern around a filament cathode 1 applied with an electron emission material so as to prevent the vibration of the cathode 1.
In FIG. 7, a modification of the electron source of FIG. 6 is shown. An electron emission material 34 is applied to a heater wire 35 defined by a straight core wire 31 wound with a coil 32 spirally to form a cathode 1', and a filament 33 is spirally wound on to the cathode 1' in an attempt to diminish the vibration of the cathode 1'.
Although the arrangement of FIG. 7 reduces the vibrations of the cathode 1', by the filament 33 spirally wrapping therearound, undesirable dust is produced as explained below. The spirally wound filament 33 is in partial contact with the electron emission material 34, which is less adhesive, and soft and spongy at a center portion thereof. Thus, the electron emission material can be easily scraped off by the filament 33. In addition, when the filament cathode vibrates, the filament 33 contacts a vibration prevention means. The contact pressure is transmitted directly to the electron emission material 34, frequently causing the electron emission material 34 to separate and peel off.